In monitoring the systolic and diastolic blood pressure of a patient by directly measuring the pressures of the maximum and minimum peaks of the blood pressure signal, the presence of equipment noise can cause fluctuations in the peak pressures that are not physiological in origin, and the presence of variations in the pressure of the thoracic cavity due to respiration can cause the peak values to slowly undulate. If the peak values are displayed on a strip chart recording or the like, the effects of such changes in their values can be discounted by the careful analysis of a skilled observer, but it is often desired to avoid the need for such analysis by displaying the systolic and diastolic values in numerical form. Fluctuations in the numerically displayed peak values would make it difficult for even a skilled observer to determine the useful systolic and diastolic pressures. This can be even more difficult if pulmonary artery pressures are being monitored because the amplitude of the undulation in pressure due to respiration can be comparable to the pulmonary artery pressures being measured.
Steady values of the systolic and diastolic blood pressures can be derived by respectively applying the fluctuating maximum and minimum peak values to suitable low pass filters, but the systolic and diastolic values derived in this manner are as much affected by peaks of noise as by peaks representing heart action. Furthermore, if the upper limit of the filter is low enough to attenuate the slow fluctuation in the peak values due to respiration, the response may be too slow to reveal sudden changes in the peak levels that are of physiological significance.